The document provides routing guidelines for efficient routing methods in TDM, ATM and IP networks. It recommends using bandwidth reservation and avoiding long paths under congestion to improve efficiency. While state-dependent routing is emphasized in ATM and IP networks, the document suggests using event-dependent routing or more efficient information dissemination to avoid inefficient flooding. It also recommends quality of service routing rules to ensure performance quality.

1. INTERNATIONAL TELECOMMUNICATION UNION
ITU-T E.352
TELECOMMUNICATION
STANDARDIZATION SECTOR
OF ITU
(03/2000)
SERIES E: OVERALL NETWORK OPERATION,
TELEPHONE SERVICE, SERVICE OPERATION AND
HUMAN FACTORS
Operation, numbering, routing and mobile services – ISDN
provisions concerning users – International routing plan
Routing guidelines for efficient routing methods
ITU-T Recommendation E.352
(Formerly CCITT Recommendation)

2. ITU-T E-SERIES RECOMMENDATIONS
OVERALL NETWORK OPERATION, TELEPHONE SERVICE, SERVICE OPERATION AND HUMAN
FACTORS
For further details, please refer to ITU-T List of Recommendations.
OPERATION, NUMBERING, ROUTING AND MOBILE SERVICES
INTERNATIONAL OPERATION
Definitions E.100–E.103
General provisions concerning Administrations E.104–E.119
General provisions concerning users E.120–E.139
Operation of international telephone services E.140–E.159
Numbering plan of the international telephone service E.160–E.169
International routing plan E.170–E.179
Tones in national signalling systems E.180–E.189
Numbering plan of the international telephone service E.190–E.199
Maritime mobile service and public land mobile service E.200–E.229
OPERATIONAL PROVISIONS RELATING TO CHARGING AND ACCOUNTING IN
THE INTERNATIONAL TELEPHONE SERVICE
Charging in the international telephone service E.230–E.249
Measuring and recording call durations for accounting purposes E.260–E.269
UTILIZATION OF THE INTERNATIONAL TELEPHONE NETWORK FOR NON-
TELEPHONY APPLICATIONS
General E.300–E.319
Phototelegraphy E.320–E.329
ISDN PROVISIONS CONCERNING USERS
International routing plan E.350–E.399
QUALITY OF SERVICE, NETWORK MANAGEMENT AND TRAFFIC ENGINEERING
NETWORK MANAGEMENT
International service statistics E.400–E.409
International network management E.410–E.419
Checking the quality of the international telephone service E.420–E.489
TRAFFIC ENGINEERING
Measurement and recording of traffic E.490–E.505
Forecasting of traffic E.506–E.509
Determination of the number of circuits in manual operation E.510–E.519
Determination of the number of circuits in automatic and semi-automatic operation E.520–E.539
Grade of service E.540–E.599
Definitions E.600–E.699
ISDN traffic engineering E.700–E.749
Mobile network traffic engineering E.750–E.799
QUALITY OF TELECOMMUNICATION SERVICES: CONCEPTS, MODELS,
OBJECTIVES AND DEPENDABILITY PLANNING
Terms and definitions related to the quality of telecommunication services E.800–E.809
Models for telecommunication services E.810–E.844
Objectives for quality of service and related concepts of telecommunication services E.845–E.859
Use of quality of service objectives for planning of telecommunication networks E.860–E.879
Field data collection and evaluation on the performance of equipment, networks and
services
E.880–E.899

3. Recommendation E.352 (03/2000) i
ITU-T RECOMMENDATION E.352
ROUTING GUIDELINES FOR EFFICIENT ROUTING METHODS
Summary
Routing policies typically used in ATM- and IP-based networks do not fully consider the possible
instabilities and drastic loss of throughput that can occur under congestion. Use of bandwidth
reservation and avoidance of long paths are recommended under such congestion, which can lead to
more efficient use of network resources. Also, there is an emphasis in ATM- and IP-based networks
on the use of state-dependent-routing (SDR) methods. However, the flooding methods typically used
by these SDR methods to disseminate network status information can lead to inefficient use of
network resources. Use of event-dependent-routing (EDR) methods and/or more efficient
dissemination of network status information are recommended as other possible approaches to
consider. Finally, QoS routing rules are recommended to ensure service performance quality, such as
avoidance of excessive transfer delay by limiting the number of satellite hops in an end-to-end
connection.
Source
ITU-T Recommendation E.352 was prepared by ITU-T Study Group 2 (1997-2000) and was
approved under the WTSC Resolution No. 1 procedure on 13 March 2000.

4. ii Recommendation E.352 (03/2000)
FOREWORD
ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of
telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of
the ITU. The ITU-T is responsible for studying technical, operating and tariff questions and issuing
Recommendations on them with a view to standardizing telecommunications on a worldwide basis.
The World Telecommunication Standardization Conference (WTSC), which meets every four years,
establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on
these topics.
The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in
WTSC Resolution No. 1.
In some areas of information technology which fall within ITU-T’s purview, the necessary standards are
prepared on a collaborative basis with ISO and IEC.
NOTE
In this Recommendation, the expression "Administration" is used for conciseness to indicate both a
telecommunication administration and a recognized operating agency.
INTELLECTUAL PROPERTY RIGHTS
The ITU draws attention to the possibility that the practice or implementation of this Recommendation may
involve the use of a claimed Intellectual Property Right. The ITU takes no position concerning the evidence,
validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others
outside of the Recommendation development process.
As of the date of approval of this Recommendation, the ITU had not received notice of intellectual property,
protected by patents, which may be required to implement this Recommendation. However, implementors are
cautioned that this may not represent the latest information and are therefore strongly urged to consult the
TSB patent database.
ã ITU 2000
All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU.

5. Recommendation E.352 (03/2000) iii
CONTENTS
Page
1 Scope........................................................................................................................... 1
2 References................................................................................................................... 1
3 Definitions .................................................................................................................. 2
4 Abbreviations.............................................................................................................. 2
5 Recommended routing methods ................................................................................. 4
5.1 Bandwidth reservation methods.................................................................................. 4
5.2 Route selection............................................................................................................ 6
5.3 QoS routing................................................................................................................. 7
6 Examples of recommended routing methods.............................................................. 8
6.1 Example of bandwidth reservation methods............................................................... 8
6.2 Example of route selection methods........................................................................... 8
Annex A − TDM-based intranetwork routing methods ........................................................... 10
A.1 Fixed routing (FR) ...................................................................................................... 10
A.2 Time-dependent routing (TDR).................................................................................. 10
A.3 State-dependent routing (SDR)................................................................................... 11
A.4 Event-dependent routing (EDR) ................................................................................. 12
Appendix I − Bibliography....................................................................................................... 12

6. iv Recommendation E.352 (03/2000)
Introduction
There are many network operators who have implemented multiple networks using different
protocols, which include Public Switched Telephone Networks (PSTNs) which use Time Division
Multiplexing (TDM) technology, Asynchronous Transfer Mode (ATM) technology, and/or Internet
Protocol (IP) technology. Various routing protocols are used in TDM-, ATM-, and IP-based
networks. In TDM-based networks, for example, Recommendation E.350 describes fixed and
dynamic routing methods for use in TDM-based networks. In ATM-based networks, for example,
the Private Network-Network Interface (PNNI) standard adopted by the ATM Forum [ATM960055]
provides for exchange of node and link status information, automatic update and synchronization of
topology databases, and dynamic route selection based on topology and status information. In IP-
based networks, for example, the open shortest path first (OSPF) and other standards adopted by the
Internet Engineering Task Force [M98] and [S95] provide for many of the same features as PNNI,
but in a connectionless IP-based packet network. OSPF also provides for exchange of node and link
status information, automatic update and synchronization of topology databases, and dynamic route
selection based on topology and status information.
This Recommendation addresses guidelines for efficient routing methods that have been studied,
learned, and implemented over many years of experience in TDM-based networks. These routing
guidelines and methods are applicable as well to ATM- and IP-based networks, and are
recommended for these networks. It is noted in the Recommendation that routing policies typically
used in ATM- and IP-based networks do not fully consider the possible instabilities and drastic loss
of throughput that can occur under congestion. Use of bandwidth reservation and avoidance of long
paths are recommended under such congestion, which can lead to more efficient use of network
resources. Also, there is an emphasis in ATM- and IP-based networks on the use of state-dependent-
routing (SDR) methods. However, the flooding methods typically used by these SDR methods to
disseminate network status information can lead to inefficient use of network resources. Use of
event-dependent-routing (EDR) methods and/or more efficient dissemination of network status
information are recommended as other possible approaches to consider. Finally, QoS routing rules
are recommended to ensure service performance quality, such as avoidance of excessive transfer
delay by limiting the number of satellite hops in an end-to-end connection.

7. Recommendation E.352 (03/2000) 1
Recommendation E.352
ROUTING GUIDELINES FOR EFFICIENT ROUTING METHODS
(Geneva, 2000)
Introduction
There are many network operators who have implemented multiple networks using different
protocols, which include Public Switched Telephone Networks (PSTNs) which use Time Division
Multiplexing (TDM) technology, Asynchronous Transfer Mode (ATM) technology, and/or Internet
Protocol (IP) technology. Various routing protocols are used in TDM-, ATM-, and IP-based
networks. In TDM-based networks, for example, Recommendation E.350 describes fixed and
dynamic routing methods for use in TDM-based networks. In ATM-based networks, for example,
the Private Network-Network Interface (PNNI) standard adopted by the ATM Forum [ATM960055]
provides for exchange of node and link status information, automatic update and synchronization of
topology databases, and dynamic route selection based on topology and status information. In IP-
based networks, for example, the open shortest path first (OSPF) and other standards adopted by the
Internet Engineering Task Force [M98] and [S95] provide for many of the same features as PNNI,
but in a connectionless IP-based packet network. OSPF also provides for exchange of node and link
status information, automatic update and synchronization of topology databases, and dynamic route
selection based on topology and status information.
This Recommendation addresses guidelines for efficient routing methods that have been studied,
learned, and implemented over many years of experience in TDM-based networks. These routing
guidelines and methods are applicable as well to ATM- and IP-based networks, and are
recommended for these networks. It is noted in the Recommendation that routing policies typically
used in ATM- and IP-based networks do not fully consider the possible instabilities and drastic loss
of throughput that can occur under congestion. Use of bandwidth reservation and avoidance of long
paths are recommended under such congestion, which can lead to more efficient use of network
resources. Also, there is an emphasis in ATM- and IP-based networks on the use of state-dependent-
routing (SDR) methods. However, the flooding methods typically used by these SDR methods to
disseminate network status information can lead to inefficient use of network resources. Use of
event-dependent-routing (EDR) methods and/or more efficient dissemination of network status
information are recommended as other possible approaches to consider. Finally, QoS routing rules
are recommended to ensure service performance quality, such as avoidance of excessive transfer
delay by limiting the number of satellite hops in an end-to-end connection.
1 Scope
This Recommendation provides guidelines for the design of routing methods within TDM-, ATM-,
and IP-based networks, and makes particular recommendations on bandwidth reservation, route
selection, and QoS routing. It recommends these guidelines based on established practice,
particularly as applied within TDM-based PSTN networks, and addresses the cases when PSTN's
evolve to incorporate IP- or ATM-based technology. Guidelines on routing methods are covered in
clause 5, and examples are given in clause 6 for the use of the routing methods.
2 References
The following ITU-T Recommendations and other references contain provisions which, through
reference in this text, constitute provisions of this Recommendation. At the time of publication, the
editions indicated were valid. All Recommendations and other references are subject to revision; all
users of this Recommendation are therefore encouraged to investigate the possibility of applying the

8. 2 Recommendation E.352 (03/2000)
most recent edition of the Recommendations and other references listed below. A list of the currently
valid ITU-T Recommendations is regularly published.
[E.164] ITU-T E.164 (1997), The International Telecommunications Numbering Plan.
[E.170] ITU-T E.170 (1992), Traffic routing.
[E.177] ITU-T E.177 (1996), B-ISDN routing.
[E.350] ITU-T E.350 (2000), Dynamic Routing Interworking.
[E.351] ITU-T E.351 (2000), Routing of multimedia connections across TDM-, ATM-, and
IP-based networks.
[E.412] ITU-T E.412 (1998), Network management controls.
[E.525] ITU-T E.525 (1992), Designing networks to control grade of service.
[E.529] ITU-T E.529 (1997), Network dimensioning using end-to-end GOS objectives.
3 Definitions
This Recommendation defines the following terms:
3.1 link: A bandwidth transmission medium between nodes that is engineered as a unit.
3.2 destination node: Terminating node within a given network.
3.3 node: A network element (switch, router/switch, exchange) providing switching and routing
capabilities, or an aggregation of such network elements representing a network.
3.4 O-D pair: An originating node to destination node pair for a given connection/bandwidth-
allocation request.
3.5 originating node: Originating node within a given network.
3.6 route: A concatenation of links providing a connection/bandwidth-allocation between an
O-D pair.
3.7 route set: A set of routes connecting the same O-D pair.
3.8 routing table: Describes the route choices and selection rules to select one route out of the
route set for a connection/bandwidth-allocation request.
3.9 traffic stream: A class of connection requests with the same traffic characteristics.
3.10 via node: An intermediate node in a route within a given network.
4 Abbreviations
This Recommendation uses the following abbreviations:
AAR Automatic Alternate Routing
ABR Available Bit Rate
AESA ATM End System Address
ARR Automatic Rerouting
AS Autonomous System
ATM Asynchronous Transfer Mode
BGP Border Gateway Protocol
B-ISDN Broadband Integrated Services Digital Network

9. Recommendation E.352 (03/2000) 3
BW Bandwidth
CAC Call Admission Control
CBR Constant Bit Rate
CCS Common Channel Signalling
DADR Distributed Adaptive Dynamic Routing
DAR Dynamic Alternate Routing
DCR Dynamically Controlled Routing
DIFFSERV Differentiated Services
DN Destination Node
DNHR Dynamic Non-Hierarchical Routing
DTL Designated Transit List
EDR Event-Dependent Routing
FR Fixed Routing
GCAC Generic Call Admission Control
GOS Grade of Service
IETF Internet Engineering Task Force
IP Internet Protocol
LLR Least Loaded Routing
LSA Link State Advertisement
LSP Label Switched Path
MPLS Multiprotocol Label Switching
N-ISDN Narrow-band Integrated Services Digital Network
ODR Optimized Dynamic Routing
ON Originating Node
OSPF Open Shortest Path First
PNNI Private Network-Network Interface
PSTN Public Switched Telephone Network
PTSE PNNI Topology State Elements
QoS Quality of Service
RP Routing Processor
RSVP Resource ReSerVation Protocol
RTNR Real-Time Network Routing
SCP Service Control Point
SDR State-Dependent Routing
STR State- and Time-Dependent Routing
TDR Time-Dependent Routing
UBR Unassigned Bit Rate

10. 4 Recommendation E.352 (03/2000)
VBR Variable Bit Rate
VC Virtual Circuit
VN Via Node
WIN Worldwide Intelligent Network (Routing)
5 Recommended routing methods
Routing policies typically used in ATM- and IP-based networks do not fully consider the possible
instabilities and drastic loss of throughput that can occur under congestion. In this clause we
recommend the use of bandwidth reservation and avoidance of long paths under such congestion to
more efficiently use network resources.
Also, there is an emphasis in ATM- and IP-based networks on the use of SDR methods. However,
the flooding methods typically used by these SDR methods to disseminate network status
information can lead to inefficient use of network resources. Use of EDR methods and/or more
efficient dissemination of network status information are recommended as other possible approaches
to consider.
Finally, QoS routing rules are recommended to ensure service performance quality, such as
avoidance of excessive transfer delay by limiting the number of satellite hops in end-to-end
connections for delay-sensitive connections to at most one hop.
5.1 Bandwidth reservation methods
Bandwidth reservation (the TDM-network terminology is "trunk reservation") gives preference to
the preferred traffic by allowing it to seize any idle bandwidth in a link, while allowing the
non-preferred routing traffic to only seize bandwidth if there is a minimum level of idle bandwidth
available, where the minimum-bandwidth threshold is called the reservation level. P. J. Burke
[Bur61] first analysed bandwidth reservation behaviour from the solution of the birth-death
equations for the bandwidth reservation model. Burke's model showed the relative lost-traffic level
for preferred traffic, which is not subject to bandwidth reservation restrictions, as compared to
non-preferred traffic, which is subject to the restrictions. Figure 1 illustrates the percentage of lost
traffic of preferred and non-preferred traffic on a typical link with 10 per cent traffic overload. It is
seen that the preferred traffic lost traffic is near zero, whereas the non-preferred lost traffic is much
higher, and this situation is maintained across a wide variation in the percentage of the preferred
traffic load. Hence, bandwidth reservation protection is robust against traffic variations and provides
significant dynamic protection of particular streams of traffic.
Bandwidth reservation is a crucial technique used in non-hierarchical networks to prevent
"instability," which can severely reduce throughput in periods of congestion, perhaps by as much as
50 per cent of the traffic-carrying capacity of a network [E.525]. The phenomenon of instability has
an interesting mathematical solution to network flow equations, which has been presented in several
studies [NaM73], [Kru82] and [Aki84]. It is shown in these studies that non-hierarchical networks
exhibit two stable states, or bistability, under congestion and that networks can transition between
these stable states in a network congestion condition that has been demonstrated in simulation
studies. A simple explanation of how this bistable phenomenon arises is that under congestion, a
network is often not able to complete a connection request on the direct or shortest route, which
consist in this example of a single link. If alternate routing is allowed, such as on longer, multiple-
link routes, which are assumed in this example to consist of two links, then the connection request
might be completed on a two-link route selected from among a large number of two-link route
choices, only one of which needs sufficient idle bandwidth on both links to be used to route the
connection. Because this two-link connection now occupies resources that could perhaps otherwise
be used to complete two one-link connections, this is a less efficient use of network resources under

11. Recommendation E.352 (03/2000) 5
congestion. In the event that a large fraction of all connections cannot complete on the direct link but
instead occupy two-link routes, the total network throughput capacity is reduced by one-half because
most connections take twice the resources needed. This is one stable state; that is, most or all
connections use two links. The other stable state is that most or all connections use one link, which is
the desired condition.
Bandwidth reservation is used to prevent this unstable behaviour by having the preferred traffic on a
link be the direct traffic on the primary, shortest route, and the non-preferred traffic, subjected to
bandwidth reservation restrictions as described above, be the alternate-routed traffic on longer
routes. In this way the alternate-routed traffic is inhibited from selecting longer alternate routes when
sufficient idle trunk capacity is not available on all links of an alternate-routed connection, which is
the likely condition under network and link congestion. Mathematically, the studies of bistable
network behaviour have shown that bandwidth reservation used in this manner to favour direct
shortest connections eliminates the bistability problem in non-hierarchical networks and allows such
networks to maintain efficient utilization under congestion by favouring connections completed on
the shortest route. For this reason, dynamic trunk reservation is universally applied in non-
hierarchical networks [E.529], and often in hierarchical networks [Mum76].
There are differences in how and when bandwidth reservation is applied, however, such as whether
the bandwidth reservation for direct-routed connections is in place at all times or whether it is
dynamically triggered to be used only under network or link congestion. This is a complex network
throughput trade-off issue, because bandwidth reservation can lead to some loss in throughput under
normal, low-congestion conditions. This loss in throughput arises because if bandwidth is reserved
for connections on the shortest route, but these calls do not arrive, then the capacity is needlessly
reserved when it might be used to complete alternate-routed traffic that might otherwise be blocked.
However, under network congestion, the use of bandwidth reservation is critical to preventing
network instability, as explained above [E.525].
5 10 15 20 25 30
14
12
10
8
6
4
2
0
T0208390-99
Average%losttraffic
Percent of preferred traffic
Preferred traffic
Non-preferred traffic
Figure 1/E.352 – Dynamic bandwidth reservation performance under 10% overload
It is recommended that bandwidth reservation techniques be included in ATM-based and IP-based
routing methods, in order to ensure the efficient use of network resources especially under
congestion conditions. Currently recommended route-selection methods, such as methods for
"Traffic Engineering" in IP-based MPLS networks [AMAOM98], or route selection in ATM-based

12. 6 Recommendation E.352 (03/2000)
PNNI networks [ATM960055], give no guidance on the necessity for using bandwidth-reservation
techniques. Such guidance is essential for acceptable network performance.
Examples are given in [A98] for dynamically triggered bandwidth reservation techniques, where
bandwidth reservation is triggered only under network congestion. Such methods are shown to be
effective in striking a balance between protecting network resources under congestion and ensuring
that resources are available for sharing when conditions permit. In [A98] the phenomenon of
network instability is illustrated through simulation studies, and the effectiveness of bandwidth
reservation in eliminating the instability is demonstrated. Bandwidth reservation is also shown to be
an effective technique to share bandwidth capacity among services integrated on a direct link, where
the reservation in this case is invoked to prefer direct link capacity for one particular service as
opposed to another service when network and link congestion are encountered. These two aspects of
bandwidth reservation, that is, for avoiding instability and for sharing bandwidth capacity among
services, are illustrated in clause 6.
In addition to the use of bandwidth reservation procedures at the time of connection request set-up, a
priority of service queuing capability is often used during the time the connection is established. For
example, at each link in an established connection, a queuing discipline is maintained such that the
packets or cells being served are given priority in some particular order, such as:
1) constant-rate services;
2) variable-rate, delay-sensitive services;
3) variable-rate, non-delay-sensitive services; and
4) variable-rate, best-effort services.
The IETF Differentiated Services (DIFFSERV) protocol [B99], for example, has queuing priorities
designated as expedited forwarding (EF), in which bandwidth can be reserved for guaranteed
throughput, and various categories of assured forwarding (AF), in which bandwidth is not reserved
or guaranteed. Use of bandwidth reservation on connection set-up, therefore, should also be linked to
bandwidth reservation used in the queuing priority discipline.
5.2 Route selection
A specific traffic routing method is characterized by the routing table used in the method. The
routing table consists of a route set and rules to select one route from the route set for a given
connection or bandwidth-allocation request. When a connection/bandwidth-allocation request is
initiated by an originating node (ON), the ON implementing the routing method executes the route
selection rules associated with the routing table for the connection/bandwidth-allocation to find an
admissible route from among the routes in the route set that satisfies the connection/bandwidth-
allocation request. In a particular routing method, the set of routes assignable to the
connection/bandwidth-allocation request may be determined according to the rules associated with
the routing table. In a network with originating connection/bandwidth-allocation control, the ON
maintains control of the connection/bandwidth-allocation request. If crankback/bandwidth-not-
available is used, for example, at a via node (VN), the preceding node maintains control of the
connection/bandwidth-allocation request even if the request is blocked on all the links outgoing from
the VN.
Routing tables consist of routes, and routes may be set up for individual connection requests such as
on switched virtual circuits (SVC). Routes may also be set up for bandwidth-allocation requests
associated with "bandwidth pipes" or "virtual trunking", such as on switched virtual paths (SVPs) in
ATM-based networks or constraint-based routing label switched paths (CRLSPs) in IP-based
networks. Routes are determined by (normally proprietary) algorithms based on the network
topology and reachable address information. These routes can cross multiple peer groups in
ATM-based networks, and multiple autonomous systems in IP-based networks, as discussed in
[E.351]. An ON may select a route from the routing table based on the routing rules and the QoS

13. Recommendation E.352 (03/2000) 7
resource management criteria, which must be satisfied on each link in the route. If a link is not
allowed based on the QoS criteria, then a release with crankback/bandwidth-not-available parameter
is used to signal that condition to the ON in order to return the connection/bandwidth-allocation
request to the ON, which may then select an alternate route.
It is recommended that route selection rules used within routing tables should allow the use of fixed
routing (FR), time-dependent routing (TDR), state-dependent routing (SDR), and event-dependent
routing (EDR) route selection, as discussed in Annex A, and the use of multilink shortest routes in a
sparse network topology. Current IP-based routing techniques, such as OSPF, and ATM-based
routing techniques, such as PNNI, emphasize SDR route selection with link-state flooding used to
convey dynamic link-status information. Typically the available-cell-rate (AvCR) is used to
determine the least-loaded-route in the SDR routing method. The least-loaded-route is the one
having the maximum available capacity across all links the route. However, the flooding of the
AvCR information on each link, which is highly variable, dynamic information, is very resource
intensive [ACFM99]. That is, significant link capacity is used to carry the flooded AvCR
information, and significant processor capacity is used to process the flooded status messages.
However, alternatives to SDR route selection are available, such as using EDR route selection
methods, or more efficient status update techniques in place of link-state flooding, such as described
in [E.350].
For instance, in one EDR route selection method, the connection/bandwidth-allocation admission
control for each link in the route is learned based on the local status of each link in a route and not on
the basis of flooded link status information. The ON normally selects the shortest route first, and
attempts to set up a connection on this route by identifying each via node (VN) in the route in the
set-up procedure. Each VN in the route then tests for available capacity on the link to the next VN. If
capacity is not available on any link in the route, the VN returns control of the connection to the ON
through a crankback/bandwidth-not-available procedure. At this point the ON then selects the last
successful alternate route, denoted as the success-to-the-top (STT) route. The STT route is tested for
available capacity in the same manner as for the shortest route. If the current STT alternate route is
not available, the ON may then selects another alternate route and tests that route for available
capacity in the same manner. That is, if a link is not allowed on the selected route, as determine by
each VN in the route based on the local link status information, then a release with
crankback/bandwidth-not-available is used to return control to the ON and select an alternate route.
The ON can check other candidate alternate routes in this way until either a new, successful STT via
is found, or the ON blocks the connection request. This EDR route selection method finds routes
through learning and local status information, and does not require the flooding of frequently
changing link-state parameters such as AvCR. This EDR approach then allows a major reduction in
the frequency of link-state flooding, and as a consequence of the reduction in the link and processor
resources consumed, allows for larger peer group sizes.
5.3 QoS routing
QoS routing constraints are recommended to be taken into account in the route selection methods.
These include end-to-end transfer delay, delay variation [G99a], and transmission quality
considerations such as loss, echo, and noise [D99], [G99a] and [G99b]. Additionally, link capability
selection [E.351] is recommended, which allows connection requests to be routed on specific
transmission media that have the particular characteristics required by these connection requests. For
example, if fibre-optic transmission is required, then only routes with links having Fibre-optic=Yes
are used. If we prefer the presence of fibre-optic transmission, then routes having all links with
Fibre-optic=Yes are used first, then routes having some links with Fibre-optic=No.
A particular QoS routing recommendation is the end-to-end transfer delay introduced by satellite
transmission. Typically, each satellite transmission link introduces about 500 milliseconds of delay,
which is above the threshold of being noticeable. Therefore, routing of delay-sensitive connections,
such as interactive voice connections, are recommended to maintain a constraint of at most one

14. 8 Recommendation E.352 (03/2000)
satellite hop in the end-to-end connection. This is typically achieved by keeping a count of the
satellite links traversed in the call set-up procedure, and inhibiting further routing on satellite links
once a single such link has been traversed.
6 Examples of recommended routing methods
In this clause we give examples of bandwidth reservation and route selection methods that might be
implemented in an ATM- or IP-based network, to illustrate the recommendations in clause 5.
6.1 Example of bandwidth reservation methods
As discussed in clause 5, bandwidth reservation can be used to favour one category of traffic over
another category of traffic. A simple example of the use of this method is to reserve bandwidth in
order to prefer traffic on the shorter primary routes over traffic using longer alternate routes. This is
most efficiently done by using a method which reserves bandwidth only when congestion exists on
links in the network. We now give an illustration of this method, and compare the performance of a
network in which bandwidth reservation is used under congestion to the case when bandwidth
reservation is not used.
In the example, traffic is first routed on the shortest route, and then allowed to alternate route on
longer routes if the primary route in not available. In the case where bandwidth reservation is used, 5
per cent of the link bandwidth is reserved for traffic on the primary route when congestion is present
on the link.
Table 1 illustrates the performance of bandwidth reservation methods for a high-day network load
pattern. In Table 1, the average business day loads for a 65-switch national network model were
inflated uniformly by 30 per cent [A98]. The table gives the average hourly lost traffic due to
blocking of connection admissions in load-set-periods 2, 3, and 5, which correspond to the two early
morning busy hours and the afternoon busy hour.
Table 1/E.352 – Performance of bandwidth reservation methods
(Percentage of lost traffic under 30% overload; 65-node network model)
Hour Without bandwidth
reservation
With bandwidth
reservation
2 12.19 0.22
3 22.38 0.18
5 18.90 0.24
We can see from the results of Table 1 that performance improves when bandwidth reservation is
used. The reason for the poor performance without bandwidth reservation is due to the lack of
reserved capacity to favour traffic routed on the more direct primary routes under network
congestion conditions. Without bandwidth reservation non-hierarchical networks can exhibit
unstable behaviour in which essentially all connections are established on longer alternate routes as
opposed to shorter primary routes, which greatly reduces network throughput and increases network
congestion [Aki84], [Kru82] and [NaM73]. If we add the bandwidth reservation mechanism, then
performance of the network is greatly improved.
6.2 Example of route selection methods
We now illustrate a comparison of state-dependent routing (SDR) in comparison to event-dependent
routing (EDR). As discussed in clause 5, use of link-state flooding to implement SDR, as is often the
case in the implementation of PNNI routing in ATM networks, or OSPF routing in IP-based
networks, can be very resource-utilization intensive. EDR is an alternative to SDR and can be

15. Recommendation E.352 (03/2000) 9
considered if the flooding overhead is deemed to be too great. As discussed in clause 5, EDR can be
implemented without the use of dynamic link-state information, and here we show that EDR
methods can still achieve good performance in comparison to SDR methods.
We now illustrate a simple comparison of SDR and EDR route selection methods. In the EDR route
selection model, the ON first routes a connection request on the shortest route. If each link has
sufficient available bandwidth according to the QoS resource management criteria, the connection is
completed. Otherwise, the ON offers the overflow from the primary shortest route to the last
successful alternate route (the success-to-the-top, or STT via route). If the connection is blocked on
the current alternate route choice, the ON selects another alternate route from the set of candidate
alternate routes. A VN uses crankback if necessary to return control to the ON if the VN finds a
selected link to have insufficient bandwidth. The ON can search through all the candidate routes
before blocking a connection request. In the SDR route selection model, the ON again routes a
connection request on the shortest route, but selects alternate routes according to link status
information. The link status is obtained by dynamic flooding of status between all network switches
as in PNNI and OSPF.
Table 2 gives performance results for a 10 per cent general overload in a 135-switch network model
in which various categories of service are modelled [ACFM99]. In the model, bandwidth reservation
is used not only to protect traffic on the primary shortest route, but also to allocate bandwidth among
the various services categories. "Key" services are given a higher priority of service than other
services under congestion, through the use of the bandwidth reservation mechanisms [A98].
Table 2/E.352 – Performance comparison of EDR and SDR route selection methods
(Percentage of lost/delayed traffic under 10% overload; 135-node network model)
Service category EDR SDR
Business-voice 1.64 1.46
Consumer-voice 1.62 1.49
International-out 3.93 5.53
International-in (key) 0.00 0.00
Key voice 0.00 0.00
64 kbit/s switched digital services 1.51 1.74
64 kbit/s ISDN data (key) 0.00 0.00
384 kbit/s ISDN data 0.00 0.00
Variable-rate delay-sensitive voice 1.09 0.41
Variable-rate non-delay-sensitive multimedia 1.01 0.38
Variable-rate best-effort multimedia 24.9 30.4
The results show the performance of the route selection methods in terms of lost traffic due to
connection admission blocking plus delayed traffic due to queuing (priority queuing was also
modelled). We can see that EDR and SDR route selection methods are quite comparable for this and
other network overload/failure scenarios modelled, and suggest that EDR is an alternative that can be
considered if the overhead of dynamic link-state flooding proves to be too resource-utilization
intensive.

16. 10 Recommendation E.352 (03/2000)
ANNEX A
TDM-based intranetwork routing methods
TDM-based routing methods described in this annex include various route selection techniques. A
specific traffic routing method is characterized by the routing table used in the method. The routing
table consists of a route and rules to select one route from the route for a given connection request.
When a connection request arrives at its ON, the ON implementing the routing method executes the
route selection rules associated with the routing table for the connection to determine a route among
the routes in the route for the connection request. In a particular routing method, the set of routes
assignable to the connection request may be altered according to a certain route alteration rule.
In Recommendations E.170, E.177, and E.350, traffic routing methods are categorized into the
following four types based on their routing pattern: fixed routing (FR), time-dependent routing
(TDR), state-dependent routing (SDR), and event-dependent routing (EDR). We discuss each of
these methods in the following clauses.
A.1 Fixed routing (FR)
In a fixed routing (FR) method, a routing pattern is fixed for a connection request. A typical example
of fixed routing is a conventional hierarchical alternate routing where the route and route selection
sequence are determined on a pre-planned basis and maintained over a long period of time. FR is
more efficiently applied when the network is non-hierarchical, or flat, as compared to the
hierarchical structure [A98].
A.2 Time-dependent routing (TDR)
Time-dependent routing (TDR) methods are a type of dynamic routing in which the routing tables
are altered at a fixed point in time during the day or week. TDR routing tables are determined on a
preplanned basis and are implemented consistently over a time period. The TDR routing tables are
determined considering the time variation of traffic load in the network. Typically, the TDR routing
tables used in the network are coordinated by taking advantage of non-coincidence of busy hours
among the traffic loads. Dynamic non-hierarchical routing (DNHR) is an example of TDR, which is
illustrated in Recommendation E.350.
In TDR, the routing tables are pre-planned and designed off-line using a centralized design system,
which employs the TDR network design model. The off-line computation determines the optimal
routes from a very large number of possible alternatives, in order to minimize the network cost. The
designed routing tables are loaded and stored in the various nodes in the TDR network, and
periodically recomputed and updated (e.g. every week) by the off-line system. In this way an ON
does not require additional network information to construct TDR routing tables, once the routing
tables have been loaded. This is in contrast to the design of routing tables in real time, such as in the
state-dependent routing and event-dependent routing methods described below. Routes in the TDR
routing table may consist of time-varying routing choices and use a subset of the available routes.
Routes used in various time periods need not be the same. Several TDR time periods are used to
divide up the hours on an average business day and weekend into contiguous routing intervals,
sometimes called load set periods.
Route selection rules employed in TDR routing tables, for example, may consist of simple sequential
routing. In the sequential method all traffic in a given time period is offered to a single route, and lets
the first route in the route overflow to the second route which overflows to the third route, and so on.
Thus, traffic is routed sequentially from route to route, and the route is allowed to change from hour
to hour to achieve the pre-planned dynamic, or time-varying, nature of the TDR method. Other TDR
route selection rules can employ probabilistic techniques to select each route in the route and thus

17. Recommendation E.352 (03/2000) 11
influence the realized flows [A98]. Routes in the TDR routing table may consist of the direct link, a
two-link route through a single VN, or a multiple-link route through multiple VNs.
A TDR connection set-up example is now given. The first step is for the node to identify the DN and
routing table information to the DN. The ON then tests for spare capacity on the first or shortest
route, and in doing this supplies the VNs and DN on this route, along with the bandwidth reservation
threshold parameter, to all nodes in the route. Each VN tests the available bandwidth capacity on
each link in the route against the bandwidth reservation threshold. If there is sufficient capacity, the
VN forwards the connection set-up to the next node, which performs a similar function. If there is
insufficient capacity, the VN sends a release message with crankback/bandwidth-not-available
parameter back to the ON, at which point the ON tries the next route in the route as determined by
the routing table rules. As described above, the TDR routes are pre-planned, loaded, and stored in
each ON.
A.3 State-dependent routing (SDR)
In state-dependent routing (SDR), the routing tables are altered automatically according to the state
of the network. For a given SDR method, the routing table rules are implemented to determine the
route choices in response to changing network status, and are used over a relatively short time
period. Information on network status may be collected at a central processor or distributed to nodes
in the network. The information exchange may be performed on a periodic or on-demand basis. SDR
methods use the principle of routing connections on the best available route on the basis of network
state information. For example, in the least loaded routing (LLR) method, the residual capacity of
candidate routes is calculated, and the route having the largest residual capacity is selected for the
connection. In general, SDR methods calculate a route cost for each connection request based on
various factors such as the load-state or congestion state of the links in the network. dynamically
controlled routing (DCR), worldwide intelligent network (WIN) routing, and real-time network
routing (RTNR) are examples of SDR, which are illustrated in Recommendation E.350.
In SDR, the routing tables are designed online by the ON or a central routing processor (RP) through
the use of network status and topology information obtained through information exchange with
other nodes and/or a centralized RP. There are various implementations of SDR distinguished by:
− whether the computation of the routing tables is distributed among the network nodes or
centralized and done in a centralized RP; and
− whether the computation of the routing tables is done periodically or connection by
connection.
This leads to three different implementations of SDR:
a) Centralized periodic SDR − Here, the centralized RP obtains link status and traffic status
information from the various nodes on a periodic basis (e.g. every 10 seconds) and performs
a computation of the optimal routing table on a periodic basis. To determine the optimal
routing table, the RP executes a particular routing table optimization procedure such as LLR
and transmits the routing tables to the network nodes on a periodic basis (e.g. every
10 seconds). DCR is an example of centralized periodic SDR, as illustrated in
Recommendation E.350.
b) Distributed periodic SDR − Here, each node in the SDR network obtains link status and
traffic status information from all the other nodes on a periodic basis (e.g. every 5 minutes)
and performs a computation of the optimal routing table on a periodic basis (e.g. every
5 minutes). To determine the optimal routing table, the ON executes a particular routing
table optimization procedure such as LLR. WIN is an example of distributed periodic SDR,
as illustrated in Recommendation E.350.

18. 12 Recommendation E.352 (03/2000)
c) Distributed call-by-call SDR − Here, an ON in the SDR network obtains link status and
traffic status information from the DN, and perhaps from selected VNs, on a connection-by-
connection basis and performs a computation of the optimal routing table for each
connection. To determine the optimal routing table, the ON executes a particular routing
table optimization procedure such as LLR. RTNR is an example of distributed connection-
by-connection SDR, as illustrated in Recommendation E.350.
Routes in the SDR routing table may consist of the direct link, a two-link route through a single VN,
or a multiple-link route through multiple VNs. Routes in the routing table are subject to DoS
restrictions on each link, and the connection setup mechanisms are similar to the example given
in A.2.
A.4 Event-dependent routing (EDR)
In event-dependent routing (EDR), the routing tables are updated locally on the basis of whether
connections succeed or fail on a given route choice. In EDR, a connection is routed first to the
shortest route, if it has sufficient available bandwidth. Otherwise, overflow from the shortest route is
offered to a currently selected alternate route. If a connection is blocked on the current alternate
route choice, another alternate route is selected from a set of available alternate routes for the
connection request according to the given EDR routing table rules. For example, the current alternate
route choice can be updated randomly, cyclically, or by some other means, and may be maintained
as long as a connection can be established successfully on the route. Note that for either SDR or
EDR, as in TDR, the alternate route for a connection request may be changed in a time-dependent
manner considering the time-variation of the traffic load. Dynamic alternate routing (DAR),
distributed adaptive dynamic routing (DADR), optimized dynamic routing (ODR), and state- and
time-dependent routing (STR) are examples of event-dependent routing, which are illustrated in
Recommendation E.350.
In EDR, the routing tables are designed by the ON using network information obtained during the
connection set-up function. Typically, the ON first selects the shortest route, and if that has
insufficient bandwidth for the connection, then the current successful via route is tried. If the current
successful via route has insufficient bandwidth, this condition is indicated by a busy ON-VN link as
determined by the ON or a busy VN-VN link or VN-DN link as indicated by a release message sent
from the VN to the ON. At that point the ON selects a new via route using the given EDR routing
table design rules. Hence, the routing table is constructed with the information determined during
connection set-up, and no additional information is required by the ON.
Routes in the EDR routing table may consist of the direct link, a two-link route through a single VN,
or a multiple-link route through multiple VNs. Routes in the routing table are subject to DoS
restrictions on each link, and the connection set-up mechanisms are similar to the example given
in A.2.
APPENDIX I
Bibliography
[A98] ASH (G.R.): Dynamic Routing in Telecommunications Networks, McGraw-Hill,
1998.
[AAFJLLS99] ASH (G.R.), ASHWOOD-SMITH (P.), FEDYK (D.), JAMOUSSI (B.), LEE (Y.),
LI (L.), SKALECKI (D.): LSP Modification Using CRLDP, draft-ash-crlsp-modify-
00.txt, July 1999.
[Aki84] AKINPELU (J.M.): The Overload Performance of Engineered Networks with Non-
hierarchical and Hierarchical Routing, Bell System Technical Journal, Volume 63,
1984.

19. Recommendation E.352 (03/2000) 13
[ACFM99] ASH (G.R.), CHEN (J.), FISHMAN (S.D.), MAUNDER (A.): Routing Evolution in
Multiservice Integrated Voice/Data Networks, International Teletraffic Congress
ITC-16, Edinburgh, Scotland, June 1999.
[ADFFT98] ANDERSON (L.), DOOLAN (P.), FELDMAN (N.), FREDETTE (A.),
THOMAS (B.): LDP Specification, IETF Draft, draft-ietf-mpls-ldp-01.txt,
August 1998.
[AL99] ASH (G.R.), LEE (Y.): Routing of Multimedia Connections Across TDM-, ATM-,
and IP-Based Networks, IETF Draft, draft-ash-itu-sg2-qos-routing-01.txt,
July 1999.
[AM98] ASH (G.R.), MAUNDER (A.): Routing of Multimedia Connections when
Interworking with PSTN, ATM, and IP Networks, AF-98-0927, Nashville TN,
December 1998.
[AAJL99] ASH (G.R.), ABOUL-MAGD (O.S.), JAMOUSSI (B.), LEE (Y.): QoS Resource
Management in MPLS-Based Networks, IETF Draft, draft-ash-qos-routing-00.txt,
Minneapolis MN, March 1999.
[AM99] ASH (G.R.), MAUNDER (A.): QoS Resource Management in ATM Networks,
AF-99-, Rome, April 1999.
[AMAOM98] AWDUCHE (D.O.), MALCOLM (J.), AGOGBUA (J.), O'DELL (M.),
McMANUS (J.): Requirements for Traffic Engineering Over MPLS, IETF Draft,
draft-ietf-mpls-traffic-eng-00.txt, October 1998.
[ATM95] B-ISDN Inter Carrier Interface (B-ICI) Specification Version 2.0 (Integrated), ATM
Forum Technical Committee, af-bici-0013.003, December 1995.
[ATM960055] Private Network-Network Interface Specification Version 1.0 (PNNI 1.0), ATM
Forum Technical Committee, af-pnni-0055.000, March 1996.
[ATM960056] Traffic Management Specification Version 4.0, ATM Forum Technical Committee,
af-tm0056.000, April 1996.
[ATM960061] ATM User-Network Interface (UNI) Signalling Specification Version 4.0, ATM
Forum Technical Committee, af-sig-0061.000, July 1996.
[ATM98] Specification of the ATM Inter-Network Interface (AINI) (Draft), ATM Forum
Technical Committee, ATM Forum/BTD-CS-AINI-01.03, July 1998.
[ATM990097] ATM Signalling Requirements for IP Differentiated Services and IEEE 802.1D,
ATM Forum, Atlanta, GA, February 1999.
[B99] BERNET (Y.), et al.: A Framework for Differentiated Services, IETF draft-ietf-
diffserv-framework-02.txt, February 1999.
[BZBHJ97] BRADEM (R.), ZHANG (L.), BERSON (S.), HERZOG (S.), JAMIN (S.): Resource
ReSerVation Protocol (RSVP) – Version 1 Functional Specification, IETF Network
Working Group RFC 2205, September 1997.
[Bur61] BURKE (P.J.): Blocking Probabilities Associated with Directional Reservation,
unpublished memorandum, 1961.
[CDFFSV97] CALLON (R.), DOOLAN (P.), FELDMAN (N.), FREDETTE (A.),
SWALLOW (G.), VISWANATHAN (A.): A Framework for Multiprotocol Label
Switching, IETF Network Working Group Draft, draft-ietf-mpls-framework-02.txt,
November 1997.
[CNRS98] CRAWLEY (E.), NAIR (R.), RAJAGOPALAN (B.), SANDICK (H.): A
Framework for QoS-based Routing in the Internet, IETF RFC 2386, August 1998.

20. 14 Recommendation E.352 (03/2000)
[D99] DVORAK (C.): IP-Related Impacts on End-to-End Transmission Performance,
ITU-T Liaison to Study Group 2, Temporary Document TD GEN-22, Geneva, May
1999.
[DN99] DIANDA (R.B.), NOORCHASHM (M.): Bandwidth Modification for UNI, PNNI,
AINI, and BICI, ATM Forum Technical Working Group, April 1999.
[G99a] GLOSSBRENNER (K.): Elements Relevant to Routing of ATM Connections,
ITU-T Liaison to Study Group 2, Temporary Document 1/2-8, Geneva, May 1999.
[G99b] GLOSSBRENNER (K.): IP Performance Studies, ITU-T Liaison to Study Group 2,
Temporary Document GEN-27, Geneva, May 1999.
[GWA97] GRAY (E.), WANG (Z.), ARMITAGE (G.): Generic Label Distribution Protocol
Specification, IETF Draft, draft-gray-mpls-generic-ldp-spec-00.txt, November
1997.
[J99] JAMOUSSI (B.), Editor: Constraint-Based LSP Setup using LDP, IETF draft-ietf-
mpls-cr-ldp-01.txt, February 1999.
[Kru82] KRUPP (R.S.): Stabilization of Alternate Routing Networks, IEEE International
Communications Conference, Philadelphia, 1982.
[LKPCD98] LUCIANI (J.), KATZ (D.), PISCITELLO (D.), COLE (B.), DORASWAMY (N.):
NBMA Next Hop Resolution Protocol (NHRP), IETF RFC 2332, April 1998.
[LR98] LI (T.), REKHTER (Y.): A Provider Architecture for Differentiated Services and
Traffic Engineering (PASTE), IETF RFC 2430, October 1998.
[M98] MOY (J.): OSPF Version 2, IETF RFC 2328, April 1998.
[Mum76] MUMMERT (V.S.): Network Management and Its Implementation on the
No. 4ESS, International Switching Symposium, Japan, 1976.
[NaM73] NAKAGOME (Y.), MORI (H.): Flexible Routing in the Global Communication
Network, Seventh International Teletraffic Congress, Stockholm, 1973.
[RVC99] ROSEN (E.), VISWANATHAN (A.), CALLON (R.): Multiprotocol Label
Switching Architecture, IETF draft-ietf-mpls-arch-04.txt, February 1999.
[S94] STEVENS (W.R.): TCP/IP Illustrated, Volume 1, The Protocols, Addison-Wesley,
1994.
[S95] STEENSTRUP (M.), Editor: Routing in Communications Networks, Prentice-Hall,
1995.
[SCFJ96] SCHULZRINNE (H.), CASNER (S.), FREDERICK (R.), JACOBSON (V.): RTP:
A Transport Protocol for Real-Time Applications, IETF RFC 1889, January 1996.
[ST98] SIKORA (J.), TEITELBAUM (B.): Differentiated Services for Internet2, Internet2:
Joint Applications/Engineering QoS Workshop, Santa Clara, CA, May 1998.
[T1S198] ATM Trunking for the PSTN/ISDN, Committee T1S1.3 (B-ISUP), T1S1.3/98, NJ,
December 1998.
[ZSSC97] ZHANG, SANCHEZ, SALKEWICZ, CRAWLEY: Quality of Service Extensions to
OSPF or Quality of Service Route First Routing (QOSPF), IETF Draft, draft-zhang-
qos-ospf-01.txt, September 1997.

22. ITU-T RECOMMENDATIONS SERIES
Series A Organization of the work of the ITU-T
Series B Means of expression: definitions, symbols, classification
Series C General telecommunication statistics
Series D General tariff principles
Series E Overall network operation, telephone service, service operation and human factors
Series F Non-telephone telecommunication services
Series G Transmission systems and media, digital systems and networks
Series H Audiovisual and multimedia systems
Series I Integrated services digital network
Series J Transmission of television, sound programme and other multimedia signals
Series K Protection against interference
Series L Construction, installation and protection of cables and other elements of outside plant
Series M TMN and network maintenance: international transmission systems, telephone circuits,
telegraphy, facsimile and leased circuits
Series N Maintenance: international sound programme and television transmission circuits
Series O Specifications of measuring equipment
Series P Telephone transmission quality, telephone installations, local line networks
Series Q Switching and signalling
Series R Telegraph transmission
Series S Telegraph services terminal equipment
Series T Terminals for telematic services
Series U Telegraph switching
Series V Data communication over the telephone network
Series X Data networks and open system communications
Series Y Global information infrastructure
Series Z Languages and general software aspects for telecommunication systems
*18125*
Printed in Switzerland
Geneva, 2000